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API Publ 2375-1997: Determination of Lean and Hydrocarbon-Rich Gas Content in Natural Gas by Gas Chromatography
Technical Overview and Implementation Guidance for Accurate Natural Gas Fractionation Analysis
Scope of API Publ 2375-1997
API Publication 2375-1997 (commonly referred to as API 2375) establishes a standardized gas chromatographic method for determining the lean gas and hydrocarbon-rich gas content of natural gas streams. Lean gas here is defined as the fraction consisting principally of methane, nitrogen, and other non-hydrocarbon components, while hydrocarbon-rich gas includes ethane, propane, butanes, pentanes and heavier hydrocarbons (C6+). The method is applicable to natural gas samples with a minimum hydrocarbon-rich gas content of 0.01 mol% and a maximum lean gas content of 99.99 mol%. It is particularly relevant for custody transfer operations, gas processing plant quality control, and pipeline blending evaluations where precise fractionation data is needed to optimize gas value and processing.
The scope of API 2375 covers both laboratory and on-line gas chromatographic configurations, provided that the analytical column set, temperature program, and detector response meet the performance criteria specified in the publication. The standard also includes guidance for sampling of single-phase gaseous mixtures from representative points, with strict avoidance of two-phase conditions that would invalidate the results.
Important: API 2375-1997 is a publication; it is not subject to the same regular updates as API standards. Users should verify that its procedures remain compatible with current industry practices and any regulatory requirements (e.g., 40 CFR Part 75).
Technical Requirements and Methodology
Chromatographic Conditions
The method requires a gas chromatograph equipped with: (a) a serial arrangement of packed or capillary columns that separate lean gases (N₂, CO₂, methane) from the hydrocarbon-rich fractions; (b) a thermal conductivity detector (TCD) for the lean gas components and a flame ionization detector (FID) for hydrocarbons; or (c) a single FID with a methanizer for carbon dioxide. Typical column configurations include a Porapak Q column for separating light hydrocarbons and a molecular sieve column for permanent gases, all maintained under a controlled temperature program from about 40°C to 220°C.
| Component Group | Typical Column | Detector | Quantitation Range (mol%) |
|---|---|---|---|
| Lean Gas (N₂, CO₂, CH₄) | Molecular Sieve 5A | TCD | 50 – 99.9 |
| Ethane, Propane | Porapak Q | FID | 0.1 – 20 |
| Butanes, Pentanes | Porapak Q or OV-1 capillary | FID | 0.01 – 10 |
| C6+ Hydrocarbons | OV-1 capillary (backflush) | FID | 0.01 – 5 |
Procedure and Calculation
After proper system conditioning and calibration using certified reference gas mixtures (traceable to NIST or equivalent), a fixed-volume gas sample is injected and components are identified by retention time. The area-percent method with relative response factors (RRFs) is used to convert detector areas to mole percent. For TCD, response factors are near unity for permanent gases; for FID, carbon-number-based RRFs are applied as provided in Appendix B of the publication. The sum of all components is normalized to 100 mol%.
Lean gas content is reported as the sum of N₂, CO₂, and CH₄; hydrocarbon-rich gas content is the sum of all hydrocarbons from C₂ to C6+. The standard specifies duplicate analyses with a within-lab repeatability of 0.05 mol% for major components and 0.01 mol% for trace components.
Tip: Backflush the heavy-end column after each analysis to avoid carryover of C6+ hydrocarbons that can bias subsequent runs. Many modern GC systems can be programmed to perform a pre-run backflush automatically.
Implementation Highlights in Field Operations
Successful implementation of API 2375 requires careful attention to sample conditioning. The standard recommends that samples be collected in stainless steel cylinders with a helium leak test, heated if necessary to maintain a single gas phase. For on-line applications, the sample system should include a coalescing filter and a pressure regulator set to deliver sample at 100–200 kPa gauge to the GC injection valve.
Calibration frequency is a key operational consideration. API 2375 mandates a full multipoint calibration (at least three concentration levels) at initial setup and whenever the column is replaced. Daily calibration verification using a single mid-range standard is acceptable, provided that the deviation from certified values is within ±2% for each component. If any component fails this check, a full recalibration is required before reporting results.
Data handling and reporting follow the format shown in the publication’s Annex A, which includes a sample calculation worksheet. The calculated gross heating value (GHV) and specific gravity of the lean and rich fractions can also be derived from the chromatographic data using standard correlations, though these are not mandatory.
Good Practice: When reporting results, include the date of the last full calibration, the serial number of the calibration standard, and the average of duplicate injections. This documentation supports both custody transfer and regulatory compliance.
Compliance and Quality Assurance Notes
API 2375-1997 does not itself include a formal quality control (QC) program, but users are expected to follow the quality guidelines of API MPMS Chapter 22 (Statistical Control of Measurement Data) and ISO 10715:2020 (Natural gas — Sampling and analysis). The standard’s performance criteria can be used as the basis for a measurement assurance program:
- Repeatability: Two consecutive results on the same sample should not differ by more than 0.10 mol% for any component above 1 mol%, and 0.02 mol% for components between 0.1 and 1 mol%.
- Reproducibility: Between-laboratory variation should not exceed 0.25 mol% for major components (above 5 mol%) and 0.05 mol% for minor components.
- Linearity: The detector response should be verified across the full working range using at least five standards.
The publication also cross-references ASTM D1945 (Standard Test Method for Analysis of Natural Gas by Gas Chromatography) and GPA 2261 (Analysis for Natural Gas and Similar Gaseous Mixtures by GC). Users may satisfy the requirements of any of these standards by following API 2375, as the methods are technically equivalent. However, specific contractual or regulatory specifications should be checked to confirm acceptance.
Caution: API 2375-1997 has not been reaffirmed since its original publication. For applications requiring a currently active standard, users should consider updating to ASTM D1945-14(2019)e1 or the latest edition of GPA 2261. Any use of API 2375 should be validated through a measurement uncertainty analysis per ISO/IEC Guide 98-3 (GUM).
Frequently Asked Questions
Q: Can API 2375-1997 be used for liquefied natural gas (LNG) vapor analysis?
A: The method is designed for single-phase gaseous samples at pressures up to 500 kPa. For LNG vapor, a heated vaporization system is required to ensure complete vaporization without condensation. With proper sampling, the method is applicable.
A: The method is designed for single-phase gaseous samples at pressures up to 500 kPa. For LNG vapor, a heated vaporization system is required to ensure complete vaporization without condensation. With proper sampling, the method is applicable.
Q: How do the results from API 2375 compare with GPA 2261?
A: The two methods are technically equivalent for the components covered. API 2375 explicitly allows the use of GPA 2261’s relative response factors and column configurations. Laboratories already following GPA 2261 can adopt API 2375 with no procedural changes.
A: The two methods are technically equivalent for the components covered. API 2375 explicitly allows the use of GPA 2261’s relative response factors and column configurations. Laboratories already following GPA 2261 can adopt API 2375 with no procedural changes.
Q: Is API 2375-1997 approved for pipeline custody transfer in North America?
A: Many contracts specify ASTM D1945 or GPA 2261, which are recognized by the North American Energy Standards Board (NAESB). API 2375 is generally accepted when validated against those standards, but it is advisable to verify acceptance with the contracting parties.
A: Many contracts specify ASTM D1945 or GPA 2261, which are recognized by the North American Energy Standards Board (NAESB). API 2375 is generally accepted when validated against those standards, but it is advisable to verify acceptance with the contracting parties.
Q: What are the main pitfalls when implementing this method on an online GC?
A: The most common issues are sample conditioning (two-phase conditions or particulates), incorrect backflush timing that can lose C6+ components, and inadequate calibration frequency for changing ambient conditions. Regular maintenance of the injection valve and columns is essential.
A: The most common issues are sample conditioning (two-phase conditions or particulates), incorrect backflush timing that can lose C6+ components, and inadequate calibration frequency for changing ambient conditions. Regular maintenance of the injection valve and columns is essential.
Article last updated: 2026