ISO 3924 Petroleum products - Determination of boiling range distribution - Gas chromatography method
5 Reagents and materials
5.1 Stationary phase for columns, non-polar, that elutes hydrocarbons in boiling-point order.

NOTE The following materials have been used successfully as liquid phases:
a) for packed columns:
silicone gum rubber UC-W98,
silicone gum rubber GE-SE-30,
silicone gum rubber OV-1,
silicone gum rubber OV-101;

b) for capillary columns:

5.2 Solid support for packed columns, usually consisting of crushed fire brick or chromatographic diatomaceous earth.

The particle size and support loading shall be such as to give optimum resolution and analysis time.

NOTE In general, support loadings of 3 % to 10 % have been found most satisfactory.

5.3 Carrier gas, consisting of helium or hydrogen for use with thermal conductivity detectors, or nitrogen, helium or argon for use with flame ionization detectors.

5.4 Calibration mixture, consisting of an accurately weighed mixture of hydrocarbons covering the range from C5 to C44 and dissolved in carbon disulfide (5.6).

The following mixture of n-alkanes has been found to be satisfactory for most samples: C5, C6, C7, C8, C9, C10, C12, C14, C16, C18, C20, C24, C28, C32, C36, C40, C44. At least one component of the mixture shall have a boiling point lower than the initial boiling point of the sample and at least one component shall have a boiling point higher than the final boiling point of the sample. The boiling points of alkanes are listed in Table 1.

NOTE It is recommended that the final concentration for packed columns be approximately 10 parts by volume of the hydrocarbon mixture to 100 parts by volume of carbon disulfide and for capillary columns, approximately 1 part by volume of the hydrocarbon mixture to 100 parts by volume of carbon disulfide.

If the test sample contains significant quantities of n-alkanes that can be identified on the chromatogram, these peaks may be used as internal boiling-point calibration points. However, it is advisable to use the calibration mixture to verify peak identifications.

Propane and butane can be added non-quantitatively to the calibration mixture, if necessary, to comply with 5.4. This may be done by bubbling a small amount of the gaseous hydrocarbon into a septum-sealed vial of the calibration mixture using a gas syringe.

If stationary phases other than those listed in 5.1, Note, are used, the retention times of a few alkylbenzenes, such as o-xylene, n-butylbenzene, 1,3,5-tri-isopropylbenzene, n-decylbenzene and n-tetradecylbenzene, across the boiling range shall also be checked to confirm that the column is separating according to the boiling point order (see Annex C).

5.5 Primary reference material, which shall be the ASTM reference gas-oil No. 1.

5.6 Carbon disulfide, reagent grade.

6 Apparatus
6.1 Chromatograph
Any gas chromatograph that has the following performance characteristics may be used.
6.1.1 Detector, of either the flame-ionization or thermal-conductivity type.
The detector shall have sufficient sensitivity to detect a mass fraction of 1.0 % (m/m) of dodecane with a peak height of at least 10 % of full scale on the recorder under the conditions specified in this International Standard, and without loss of resolution as defined in 8.3. When operating at this sensitivity level, the detector stability shall be such that a baseline drift of not more than 1 % of full scale per hour is obtained. The detector shall be capable of operating continuously at a temperature equivalent to the maximum column temperature employed. The detector shall be connected to the column in such a way that cold spots between the detector and the column are avoided.

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

6.1.2 Column temperature programmer, capable of programmed temperature operation over a range sufficient to establish a retention time of at least 1 min for the initial boiling point and to elute the entire sample within the temperature ramp.

The programming rate shall be sufficiently reproducible to obtain retention time repeatability of 6 s for each component in the calibration mixture (5.4).

If the initial boiling point is less than approximately 93 °C, an initial column temperature below ambient can be required. However, excessively low initial column temperatures shall be avoided to ensure that the stationary phase remains liquid. The initial temperature of the column shall be only low enough to obtain a calibration curve meeting the requirements of this International Standard.

6.1.3 Sample inlet system, either be capable of operating continuously at a temperature equivalent to the maximum column temperature employed or provide on-column injection with some means of programming the entire column, including the point of sample introduction, up to the maximum temperature required.

The sample inlet system shall be connected to the chromatographic column in such a way that cold spots between the inlet system and the column are avoided.

6.2 Column
Any column and conditions may be used, provided that, under the conditions of the test, separations are in the order of boiling points as given in Table 1, and that the column resolution, R, is at least 3. Typical column operating conditions are given in Tables 2 and 3.

6.3 Recorder/plotter
This apparatus is used for plotting the chromatogram. This may be accomplished using a 0 mV to 1 mV recording potentiometer having a full-scale response time of 2 s or less and a minimum chart width of approximately 120 mm. Alternatively, a computer or other device may be used, provided it is capable of a graphics presentation of the same or better quality as a potentiometric recorder.

6.4 Integrator/computer
This apparatus is used for determining the accumulated area under the chromatogram. This may be achieved by using a computer-based chromatography data system or an electronic integrator. The integrator/computer system shall have normal chromatographic software for measuring the retention times and areas of eluting peaks. In addition, the system shall be capable of converting the continuously integrated detector signal into area slices of fixed duration. These contiguous area slices, collected for the entire analysis, shall be stored for later processing. The electronic range of the integrator/computer (e.g. 1 V) shall be within the linear range of the detector/electrometer system used. The system shall be capable of subtracting the area slice of a blank run from the corresponding area slice of a sample run.

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

6.5 Flow/pressure controllers
6.5.1 If a packed column is used, the chromatograph shall be equipped with constant-flow controllers capable of maintaining the carrier gas flow constant to +/- 1 % over the full operating temperature range.

6.5.2 If a wide-bore capillary column is used, the chromatograph shall be equipped with a controller of carrier gas flow or pressure appropriate for the inlet used.

6.6 Micro-syringe
This apparatus is used to introduce the sample into the chromatograph.

Sample injection may be either manual or automatic. Automatic sample injection is preferred because it gives better retention time precision.