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D2789-95 – Standard Test Method Technical Guide
📐 Scope and Applicable Fuel Types
ASTM D2789‑95 (Reapproved 2016) specifies a standard test method for determining hydrocarbon types in gasoline using mass spectrometry. The method is specifically designed for low‑olefin gasoline containing less than 3 % by volume of olefins and having a 95 % distillation point below 210 °C (411 °F), as determined by Test Method D86.
⚠️ Important Limitation: The applicability of this test method has not been established for gasoline containing oxygenated compounds such as alcohols and ethers (Section 1.2). Analysts testing oxygenated blends must rely on validated alternative methods.
The method determines six distinct hydrocarbon groups: total paraffins, monocycloparaffins, dicycloparaffins, alkylbenzenes, indans / tetralins, and naphthalenes.
⚙️ Test Methodology and Fragment Analysis
Samples are analyzed via mass spectrometry, relying on the summation of characteristic mass fragments (m/e). The concentration of each hydrocarbon type is derived from the intensity of specific ion fragments defined in Section 3.1.1 of the standard. The total ion intensity T represents the sum of all fragment groups.
💡 Core Equation: The total ion intensity T is calculated as:
T = Σ(43) + Σ(41) + Σ(67) + Σ(77) + Σ(103) + Σ(128)
⚡ Characteristic Mass Fragment Summations
The following table defines the mass‑to‑charge ratios (m/e) used to identify each hydrocarbon type.
| 🟦 Hydrocarbon Type | 📏 Summation Notation | 🎯 Key Mass Fragments (m/e) |
|---|---|---|
| Paraffins | Σ(43) | 43, 57, 71, 85, 99 |
| Monocycloparaffins | Σ(41) | 41, 55, 69, 83, 97 |
| Dicycloparaffins | Σ(67) | 67, 68, 81, 82, 95, 96 |
| Alkylbenzenes | Σ(77) | 77, 78, 79, 91, 92, 105, 106, 119, 120, 133, 134, 147, 148, 161, 162 |
| Indans & Tetralins | Σ(103) | 103, 104, 117, 118, 131, 132, 145, 146, 159, 160 |
| Naphthalenes | Σ(128) | 128, 141, 142, 155, 156 |
📊 Key Measured Properties and Supporting Standards
The accuracy of the hydrocarbon type determination relies on precise distillation characteristics and supporting analytical tests. The table below lists the critical ASTM standards referenced in D2789‑95.
| 📐 Standard | ⚡ Title and Relevance |
|---|---|
| D86 | Distillation of Petroleum Products (defines the 210 °C distillation cutoff point) |
| D1319 | Hydrocarbon Types by Fluorescent Indicator Adsorption (primary olefin determination) |
| D2001 | Depentanization of Gasoline and Naphthas (sample preparation procedure) |
| D875 (Withdrawn 1984) | Calculating Olefins and Aromatics from Bromine Number (historical alternative) |
The average number of carbon atoms in the sample (carbon number) is another critical parameter defined in Section 3.1.2, governing the calibration and final calculation of the hydrocarbon type distribution.
❓ Frequently Asked Questions
🔍 What is the maximum olefin content allowed for a gasoline to be tested by this method?
The standard explicitly limits its scope (Section 1.1) to gasolines with an olefin content of less than 3 % by volume. Higher olefin concentrations can cause interference in the mass spectral fragmentation patterns and invalidate the results.
💡 Can ASTM D2789 be applied to gasoline containing ethanol or ethers?
No. Section 1.2 of the standard states: “It has not been determined whether this test method is applicable to gasoline containing oxygenated compounds.” Analysts must use methods validated for oxygenated fuel blends.
⚡ How does the method define “carbon number”?
According to Section 3.1.2, the carbon number is defined as the average number of carbon atoms in the sample. This estimated value is essential for converting the raw ion intensity data into accurate hydrocarbon type concentrations.
📌 What is the significance of the total ion intensity (T) calculation?
The total ion intensity T, defined as the sum of the six characteristic mass fragment groups (Σ(43) through Σ(128)), represents the entire target spectrum. It serves as the normalization factor against which the individual hydrocarbon group concentrations are calculated (Section 3.1.1).