ASTM D2887 Boiling Range Distribution of Petroleum Fractions by Gas Chromatography
6. Apparatus
6.1 Chromatograph - The gas chromatograph used must have the following performance characteristics:
6.1.1 Detector - Either a flame ionization or a thermal conductivity detector may be used. The detector must have sufficient sensitivity to detect 1.0 % dodecane with a peak height of at least 10 % of full scale on the recorder under conditions prescribed in this test method and without loss of resolution as defined in 9.3.1. When operating at this sensitivity level, detector stability must be such that a baseline drift of not more than 1 % of full scale per hour is obtained. The detector must be capable of operating continuously at a temperature equivalent to the maximum column temperature employed. Connection of the column to the detector must be such that no temperature below the column temperature exists.

NOTE 1 - It is not desirable to operate a thermal conductivity detector at a temperature higher than the maximum column temperature employed. Operation at higher temperature generally contributes to higher noise levels and greater drift and can shorten the useful life of the detector.

6.1.2 Column Temperature Programmer - The chromatograph must be capable of linear programmed temperature operation over a range sufficient to establish a retention time of at least 1 min for the IBP and to elute compounds up to a boiling temperature of 538°C (1000°F) before reaching the upper end of the temperature program. The programming rate must be sufficiently reproducible to obtain retention time repeatability of 0.1 min (6 s) for each component in the calibration mixture described in 7.8.

6.1.3 Cryogenic Column Cooling - Column starting temperatures below ambient will be required if samples with IBPs of less than 93°C (200°F) are to be analyzed. This is typically provided by adding a source of either liquid carbon dioxide or liquid nitrogen, controlled through the oven temperature circuitry. Excessively low initial column temperature must be avoided to ensure that the stationary phase remains liquid. The initial temperature of the column should be only low enough to obtain a calibration curve meeting the specifications of the method.

6.1.4 Sample Inlet System - The sample inlet system must be capable of operating continuously at a temperature equivalent to the maximum column temperature employed, or provide for on-column injection with some means of programming the entire column, including the point of sample introduction, up to the maximum temperature required. Connection of the column to the sample inlet system must be such that no temperature below the column temperature exists.

6.1.5 Flow Controllers - The gas chromatograph must be equipped with mass flow controllers capable of maintaining carrier gas flow constant to ±1 % over the full operating temperature range of the column. The inlet pressure of the carrier gas supplied to the gas chromatograph must be sufficiently high to compensate for the increase in column back-pressure as the column temperature is raised. An inlet pressure of 550 kPa (80 psig) has been found satisfactory with the packed columns described in Table 1. For open tubular columns, inlet pressures from 10 to 70 kPa (1.5 to 10 psig) have been found to be suitable.

6.1.6 Microsyringe - A microsyringe is needed for sample introduction.

NOTE 2 - Automatic sampling devices or other sampling means, such as indium encapsulation, can be used provided: the system can be operated at a temperature sufficiently high to completely vaporize hydrocarbons with atmospheric boiling points of 538°C (1000°F), and the sampling system is connected to the chromatographic column avoiding any cold temperature zones.

6.2 Column - Any column and conditions may be used that provide separation of typical petroleum hydrocarbons in order of increasing boiling point and meet the column performance requirements of 9.3.1 and 9.3.3. Successfully used columns and conditions are given in Table 1.

6.3 Data Acquisition System:
6.3.1 Recorder - A 0 to 1 mV range recording potentiometer or equivalent, with a full-scale response time of 2 s or less may be used.

6.3.2 Integrator - Means must be provided for determining the accumulated area under the chromatogram. This can be done by means of an electronic integrator or computer-based chromatography data system. The integrator/computer system must have normal chromatographic software for measuring the retention time and areas of eluting peaks (peak detection mode). In addition, the system must be capable of converting the continuously integrated detector signal into area slices of fixed duration. These contiguous area slices, collected for the entire analysis, are stored for later processing. The electronic range of the integrator/computer (for example, 1 V, 10 V) must be within the linear range of the detector/electrometer system used. The system must be capable of subtracting the area slice of a blank run from the corresponding area slice of a sample run.

NOTE 3 - Some gas chromatographs have an algorithm built into their operating software that allows a mathematical model of the baseline profile to be stored in memory. This profile is automatically subtracted from the detector signal on subsequent sample analyses to compensate for any baseline offset. Some integration systems also store and automatically subtract a blank analysis from subsequent analytical determinations.

7. Reagents and Materials
7.1 Purity of Reagents - Reagent grade chemicals shall be used in all tests. Unless otherwise indicated, it is intended that all reagents conform to the specifications of the Committee on Analytical Reagents of the American Chemical Society where such specifications are available. Other grades may be used, provided it is first ascertained that the reagent is of sufficiently high purity to permit its use without lessening the accuracy of the determination.

7.2 Liquid Phase for Columns - Methyl silicone gums and liquids provide the proper chromatographic hydrocarbon elution characteristics for this test method.

7.3 Solid Support for Packed Columns - Chromatographic grade diatomateous earth solid support material within a particle size range from 60 to 100 sieve mesh size is recommended.

7.4 Carrier Gas - Helium or nitrogen of high purity. (Warning - Helium and nitrogen are compressed gases under high pressure.) Additional purification is recommended by the use of molecular sieves or other suitable agents to remove water, oxygen, and hydrocarbons. Available pressure must be sufficient to ensure a constant carrier gas flow rate (see 6.1.5).

7.5 Hydrogen - Hydrogen of high purity (for example, hydrocarbon free) is used as fuel for the flame ionization detector (FID). (Warning - Hydrogen is an extremely flammable gas under high pressure.)

7.6 Air - High purity (for example, hydrocarbon free) compressed air is used as the oxidant for the flame ionization detector (FID). (Warning - Compressed air is a gas under high pressure and supports combustion.)

7.7 Column Resolution Test Mixture - For packed columns, a nominal mixture of 1 mass % each of n-C16 and n-C18 paraffin in a suitable solvent, such as n-octane, for use in testing the column resolution. (Warning - n-octane is flammable and harmful if inhaled.) The calibration mixture specified in 7.8.2 may be used as a suitable alternative, provided the concentrations of the n-C16 and n-C18 components are nominally 1.0 mass % each. For open tubular columns, use the mixture specified in 7.8.3.

7.8 Calibration Mixture - An accurately weighed mixture of approximately equal mass quantities of n-hydrocarbons dissolved in carbon disulfide (CS2). (Warning - Carbon disulfide is extremely volatile, flammable, and toxic.) The mixture shall cover the boiling range from n-C5 to n-C44, but does not need to include every carbon number (see Note 4).
7.8.1 At least one compound in the mixture must have a boiling point lower than the IBP of the sample and at least one compound in the mixture must have a boiling point higher than the FBP of the sample. Boiling points of n-paraffins are listed in Table 2.
7.8.1.1 If necessary, for the calibration mixture to have a compound with a boiling point below the IBP of the sample, propane or butane can be added to the calibration mixture, non-quantitatively, by bubbling the gaseous compound into the calibration mixture in a septum sealed vial using a gas syringe.

NOTE 4 - Calibration mixtures containing normal paraffins with the carbon numbers 5, 6, 7, 8, 9, 10, 11, 12, 14, 15, 16, 17, 18, 20, 24, 28, 32, 36, 40, and 44 have been found to provide a sufficient number of points to generate a reliable calibration curve.

7.8.2 Packed Columns - The final concentration should be approximately ten parts of the n-paraffin mixture to one hundred parts of CS2.

7.8.3 Open Tubular Columns - The final concentration should be approximately one part of the n-paraffin mixture to one hundred parts of CS2.

7.9 Reference Gas Oil No. 1 - A reference sample that has been analyzed by laboratories participating in the test method cooperative study. Consensus values for the boiling range distribution of this sample are given in Table 3.

8. Sampling
8.1 Samples to be analyzed by this test method must be obtained using the procedures outlined in Practice D4057.

8.2 The test specimen to be analyzed must be homogeneous and free of dust or undissolved material.